1. Field of the Invention
The present invention relates to a method of preventing to form a spacer undercut, and more particularly to a method of etching the oxide and nitride spacer simultaneously to prevent from forming a spacer undercut in SEG Pre-clean process.
2. Description of the Prior Art
Generally, as semiconductor dimensions continue to shrink and device densities increase, contact resistance and junction depth become increasingly critical for device performance. Raised source and drain structures can provide shallow junctions with low series resistance, enhancing performance. Raised source and drain structures are typically fabricated using selective epitaxial growth (SEG) method which need a clean surface of silicon substrate; however, the surface of silicon substrate is accessible to form a native oxide with aqueous and oxygen atom from the air, for instance a silicon dioxide layer. Therefore, it is generally utilized hydrofluoric acid (HF) solution to remove the native oxide on the silicon substrate. However, employing hydrofluoric acid in order to remove the oxide on the silicon substrate that will produce a spacer undercut and result in leakage current between source, drain and gate.
The formation of an undercut is due to the wet etching. A thin film will be generated two kinds of profile after etching process, which is isotropic and anisotropic etching profile respectively. Also, the wet etching is belonged to isotropic etching and chemical reaction that does not have any direction when performing a reaction. It will produce lateral and vertical etching simultaneously; therefore, the undercut is created.
Typically, the Pre-clean process before performing SEG in raised source and drain modules is described as in FIG. 1A and FIG. 1B. Referring to FIG. 1A, a semiconductor substrate 101 is provided firstly; for instance a p-type or n-type silicon substrate. Then, a plurality of isolation is formed in the semiconductor substrate 101; for instance a plurality of isolation is formed by shallow trench isolation (STI) process. A gate oxide 105 and a gate electrode 107 are sequentially formed between the plurality of isolation. Next, a spacer 109 of the double-film structure is formed on the side-wall gate oxide 105 and side-wall gate electrode 107; wherein the spacer 109 comprises a silicon dioxide layer 109A and a silicon nitride layer 109B. Referring to FIG. 1B, an epi-layer is formed subsequently as raised source and drain 113 on the exposed semiconductor substrate 101 by selective epitaxial growth (SEG) and chemical vapor deposition (CVD) method. However, it need the clean surface of the semiconductor substrate 101 prior to forming the epi-layer by SEG technique so that a way of wet etching, which utilizes DHF (HF in deionized water), will remove the native oxide on the semiconductor substrate 101. As a result, the hydrofluoric acid; wherein the HF is diluted in deionized water ( 1/100 in volume %) silicon dioxide layer 109A of the spacer 109. Therefore, an undercut 111 will be created within the silicon dioxide spacer. Also, after raised source/drain 113 is deposited by SEG technique, the undercut 111 will generate defects between raised source/drain 113 and gate 107, further; leakage current will be produced. In addition, the epi-layer will be selectively grown under the spacer 109 and toward to gate electrode 107; therefore, it will result in bridge effect after forming salicide between raised source/drain 113 and gate 107.
Due to the fact that utilizes a hydrofluoric acid solution in order to remove the native oxide, a spacer undercut is created and result in leakage current between source, drain and gate. Therefore, the present invention provides a method for preventing to form a spacer undercut in SEG Pre-clean process.